1. Field of the Invention
The present invention relates to an anode active material and a lithium battery using the same, and more particularly, to an anode active material having a large capacity and excellent capacity retention and a lithium battery having a long cycle life using the same.
2. Description of the Related Art
When lithium metal is used as the anode active material in lithium batteries, short-circuits may occur due to the formation of dendrites, and a risk of explosion results. Accordingly, carbon-based materials are widely used as anode active materials instead of lithium metal.
Examples of carbon-based active materials used as anode active materials in lithium batteries include crystalline carbon, such as natural graphite and artificial graphite, and amorphous carbon, such as soft carbon and hard carbon. Although amorphous carbon has good capacity, when amorphous carbon is used, many of the charge/discharge reactions are irreversible. Natural graphite is the most commonly used crystalline-based carbon, and the theoretical capacity of natural graphite can be as high as 372 mAh/g. Therefore, crystalline carbon is widely used as the anode active material. However, although the 372 mAh/g theoretical capacity of such carbon-based active materials (including graphite) is currently considered to high, this capacity may not be sufficient for future lithium batteries which may require higher capacities.
Accordingly, research into metal-based anode active materials and intermetallic compound-based anode active materials has been actively conducted. For example, research into lithium batteries using metals or semimetals such as aluminum, germanium, silicon, tin, zinc, lead, etc, as the anode active materials has been conducted. Such materials are known to have large capacities, high energy densities, and good insertion/extraction capabilities compared to carbon-based anode active materials. Thus, lithium batteries having large capacities and high energy densities can be prepared using these materials. For example, pure silicon is known to have a high theoretical capacity of 4017 mAh/g.
However, such materials have shorter cycle lives than carbon-based materials, and thus cannot be put to practical use. When inorganic particles such as silicon or tin are used as the anode active material, the volume of inorganic particles changes considerably during charge/discharge cycles. This may result in the degradation of the electronic conduction network between the active material particles or may result in the detachment of the anode active material from the anode current collector, as shown in FIG. 1. That is, the volume of inorganic material such as silicon or tin increases by about 300 to 400% due to alloying with lithium during charging, and the volume decreases due to extraction of lithium during discharging. Therefore, after repeated charge/discharge cycles, spaces may be generated between the active material particles and electrical insulation may occur, thereby causing the capacity of the battery to rapidly decrease.
To address these concerns, an anode active material including a mixture of flakes of metal or alloy powder, flakes of carbon powder, and binder has been produced. However, since the mixture is obtained by simple mixing, stress occurs in the battery due to the expansion and contraction of particles upon repeated charging and discharging, resulting in a serious interruption of electronic conduction.
In addition, a lithium secondary battery in which nanoparticles of metals are coated with carbon has also been produced. However, the carbon (which is brittle) on the surface of the nanoparticles cracks due to expansion during charging, and spaces are generated between carbon and metal nanoparticles due to contraction during discharging. Therefore, improvements in battery cycle life are restricted.
Therefore, a need exists for an anode active material with high capacity and good capacity retention, and for a lithium battery with a long cycle life employing such an anode active material.